A PON system consists of a plurality of optical network units (home side apparatuses) and an optical line terminal connected thereto through optical fibers. The optical line terminal is equipped with clock and data recovery circuits that extract clock signals and data signals from upstream signals transmitted from the optical network units (see, for example, Patent Document 1).
A clock and data recovery circuit includes a voltage-controlled oscillator (VCO) that outputs an oscillation frequency according to a control voltage. The output characteristics of the voltage-controlled oscillator are slightly changed by variations in element characteristics or by temperature change. Hence, there is proposed an oscillator in which variable capacitors for compensating the element characteristics are provided to adjust capacitance to compensate for variations in element characteristics, and for temperature, variable capacitors for compensation are separately provided to perform temperature compensation by changing capacitance based on information from a temperature detector (see, for example, Patent Document 2).